This invention relates to an exposure apparatus in which the illuminating light is ultraviolet radiation or excimer laser light that has a particularly strong luminous flux and readily activates an atmospheric gas.
An exposure apparatus used in the manufacture of semiconductors and the like is required to have higher throughput and resolution owing to much smaller pattern line width. This has been accompanied by the need for exposing light having ever higher illuminance. At the same time, the wavelength of the exposing light is becoming increasingly short.
However, it is known that exposing light causes impurities in the air to react photochemically with oxygen in an exposure apparatus that uses i lines (wavelength xcex=365 nm) as the exposing light and in an exposure apparatus that uses exposing light, the wavelength of which is shorter than that of i lines. Such reaction products (fogging substances) attach themselves to glass members and produce an opaque fog on the glass members. A typical example of such a fogging substance is ammonium sulfate (NH4)2SO4, which is produced by a reaction (oxidation) with oxygen in the air, if sulfur dioxide (SO2) absorbs the energy of the exposing light and attains the excited state. The ammonium sulfate is white in color and results in fogging when it attaches itself to the surfaces of optical members such as lenses and mirrors. The exposing light is scattered and absorbed by the ammonium sulfate, as a consequence of which, there is a decline in the transmittance of the optical system.
In the region of short wavelengths in which the exposing light has a wavelength of less than 248 [nm], which is shorter than that of i lines, as in the case of a KrF excimer laser, the exposing light causes an even stronger photochemical reaction. As a result, not only does the above-mentioned fogging occur but so does a phenomenon in which the exposing light reacts with oxygen in the air to produce ozone, with both residual oxygen and the generated ozone absorbing the exposing light.
Accordingly, a method of preventing the contamination of optical members has been developed. The method involves placing the optical system, such as the lens of the light source or the projecting lens system, within a vessel and then replacing the air in the vessel with an inert gas such as nitrogen gas or with another gas from which impurities have been removed, whereby the aforementioned contamination is avoided.
In recent years, however, the illuminating systems of exposure apparatus have become increasingly complex in order to achieve diverse illuminating conditions and deformed illumination. As a consequence, maintenance is essential and the spaces filled with the inert gas frequently are exposed to the outside atmosphere whenever such maintenance is carried out. In addition, the inert gas used, such as nitrogen or helium, may have an influence upon the human body. This means that while maintenance is being performed, the supply of the inert gas must be stopped in order to assure safety. A consequence of this is that the interior of the vessel becomes filled with atmospheric air during maintenance work. After maintenance, the atmospheric air within the vessel must be replaced with inert gas again. Furthermore, if the exposure apparatus is out of action for a long period of time, it is likewise necessary for the inert gas to be substituted.
However, the areas filled with the inert gas inside the illuminating system and projecting lens system have become increasingly complex in structure and it is difficult for the inert gas to flow when the substitution is made from atmospheric air to the inert gas. Many points of stagnation result. Air accumulates at the stagnation points and the air accumulated at such points flows out only in small increments. As a consequence, the time needed for air within the vessel to be fully replaced with the inert gas is prolonged. The structure internally of the projecting lens system is particularly complicated and does not lend itself to the incorporation of a mechanism that prevents the accumulation of air within the vessel.
Accordingly, an object of the present invention is to provide an exposure apparatus in which the air within the vessel of an illuminating system and within the vessel of projection lenses can be replaced with an inert gas highly efficiently, thereby minimizing the time needed to make the substitution from atmospheric air to the inert gas.
According to the present invention, the foregoing object is attained by providing an exposure apparatus for illuminating a reticle with exposing light from an exposing light source via an illuminating optical system and projecting a pattern, which has been formed on the reticle, onto a substrate via a projection optical system, comprising: a first vessel within which the illuminating optical system is placed; a second vessel within which the projection optical system is placed; the first vessel having an inlet and an outlet for a first gas and the second vessel having an inlet and an outlet for a second gas; substitution means for substituting the first gas and/or the second gas for a gas in the interior of the first vessel and/or the second vessel; and exhaust means for evacuating the first vessel and/or the second vessel in order to establish negative pressure in the interior thereof when the first gas and/or the second gas is substituted for the gas in the interior of the first vessel and/or the second vessel.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.